With normal vision, an individual is able to change focus for different distances. Ideally, an individual is able to focus on distant objects, referred to as distance vision, and on near objects, referred to as near vision. The optical system of the eye uses numerous muscles to focus for both distance and near vision. These muscles allow the eye to adjust focus when transitioning between distance vision and near vision. There are various responses involved in changing focus from distance vision to near vision. These include making the image clearer, the eyes turning in or out, and pupils changing size. If the eyes do not turn in enough with near vision, for example, then the individual would see double.
Single vision lenses have been used for many years to correct farsightedness, nearsightedness, and astigmatism (ametropia). When we look directly through the optical center of a single vision lens there is no prismatic effect induced in any direction. As your eyes move away from the optical center of a lens (correcting ametropia) prism results and increases as you move farther from the optical center. This prism is mostly induced by the curvature difference in the two major lens surfaces and is the result of moving away from the optical center. The amount of prism changes with both power and distance from the optical center. The stronger the lens and the farther from the optical center the more prism is induced. For someone that is farsighted wearing a lens to correct hyperopia, converging to a near object causes the eyes to look towards the nasal side of the optical center of the lenses. In this instance base out prism would be induced when looking at near therefore causing an increase in the convergence demand necessary to view the object under binocular conditions. This increased convergence can be a source of asthenopia.
Bifocals, including progressive addition lenses (PALs), have been used for many years to help with presbyopia. PALs provide the ability for a person wearing these type of lenses to see at different distances. PALs or Bifocals are generally needed when the eye can no longer make the total change in focus necessary from distance to near. PALs typically provide a distance portion in the lens (this would include the patient's prescription to see clearly at a distance) and a smooth graduating continuous change in dioptric power to a near power (this would be the diopter add power for the patient to see clearly at near). PALs have no lines or edges visible between changes in dioptric power. PALs are available for example in different add powers, corridor lengths and corridor widths depending on the need.
Prior art PALS thus traditionally include undesired base out prism that is induced at near when the optical surface of the lens is altered to create the add power at near.